Wind turbines are used to produce electrical power from the energy contained in blowing wind. The wind turbine comprises a rotor which is driven by the wind and which in turn drives an induction generator, which usually is an AC generator.
When the wind turbine is connected to a utility grid which requires a certain frequency, e.g. 50 Hz in Europe, the electrical power output of the wind turbine needs to be synchronized to this frequency. As modern wind turbines often work with variable rotational speed of the rotor and thus of the AC induction generator a conversion from the frequency produced in the generator to the frequency of the utility grid is necessary. This conversion is usually performed by transforming the AC voltage delivered by the generator into a DC voltage which is then transformed again into an AC voltage with fixed frequency. The AC voltage will then be fed into the utility grid.
To keep the frequency and the voltage fed into the utility grid in tolerable ranges, control systems are used for controlling this values. For the calculations performed with regard to such controlling the knowledge of the rotational speed and the phase angle of the rotor, i.e. the angle of a radial line extending from the rotor's center and rotating together with the rotor relative to a non-rotating reference line through the center of the rotor, is very useful. Variable speed wind turbines having a control mechanism for controlling the voltage fed into the utility grid are, e.g., disclosed in WO 2005/091490 A1 or U.S. Pat. No. 5,083,039.
The rotational speed of the rotor and the phase angle of the rotor are usually measured inside the nacelle where the rotational speed is measured at a low-speed or high-speed shaft of the rotor by an inductive sensor. The phase angle of the rotor is usually measured by an absolute encoder placed at the end of the rotor's slip ring. However, in these measurements the speed and the phase angle are measured in a local frame of reference, i.e. relative to the position of the wind turbines tower top. This leads to measurement errors if the tower top is moving. These measurement errors introduce an apparent cyclic oscillation of the measured rotor speed that is only in artefact. Any control mechanisms based on such measurement values include the risk of introducing artificial control requirements. Moreover the speed measurement at the high-speed shaft lacks information on the phase angle of the rotor and may include torsion oscillations.